A Voltage Controlled Oscillator is an electronic circuit capable of generating an output signal of a certain nominal frequency Farr, and wherein the output frequency can be controlled by changing the value of frequency control voltage applied to the frequency control voltage input.
In Voltage Controlled Crystal Oscillators (VCXOs), the frequency control voltage is applied to a voltage controlled variable capacitance (“varactor”) or a circuit functionally equivalent to a varactor; when the frequency control voltage value changes, the varactor's capacitance changes too, causing in turn the oscillator's frequency FOUT to change, thus effecting the desired output frequency control function.
The frequency versus voltage transfer function of a VCXO is usually non-linear due to non-linearity of the varactor's capacitance versus voltage transfer function, as well as non-linearities of the oscillator circuit. Moreover, the shape of VCXOs' frequency versus voltage transfer function is different for different VCXO samples.
VCXOs find application in the implementation of many electronic circuit functions and devices, including but not limited to Phase Lock Loops (PLLs), electronic frequency multipliers, Temperature Compensated Crystal Oscillators (TCXOs), etc.
In at least some applications, it is desirable that a Voltage Controlled Oscillator exhibits a particular and consistent shape of its effective frequency versus voltage transfer function. Often, a linear shape is required. An example of such an application is a type of crystal oscillator called Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO). In many contemporary VCTCXOs, temperature compensation is achieved by generating a temperature-dependent compensation voltage and applying it to an internal Voltage Controlled Crystal Oscillator (VCXO). The structure of such VCTCXO devices is shown in FIG. 1 (prior art). In this diagram, the VCXO is a tunable circuit that produces an output signal of required frequency FOUT. The output frequency FOUT is temperature-dependent. In order to improve the frequency Farr versus temperature stability, a Temperature Compensation Function Generator is used to produce temperature compensating voltage VCOMP, and the latter is applied to the VCXO to correct the output frequency for the effects of ambient temperature changes. The compensating voltage is produced as a function of the Temperature Sensor output signal and the function is tailored so that, in conjunction with the VCXO's frequency versus temperature characteristic, the application of the VCOMP voltage reduces the output frequency FOUT instability caused by ambient temperature changes.
User-controlled VCO voltage allows the VCTCXO user to tune the VCTCXO device by changing the VCO voltage level. The temperature compensation voltage VCOMP generated by the Temperature Compensation Function Generator is usually optimized at a certain fixed level of the externally applied VCO voltage, such as the midpoint of the VCO voltage range. This means that the VCOMP voltage is optimized for a single value of the VCXO's frequency versus voltage characteristic slope. When the user-controlled VCO voltage is changed to a different value within the VCO range, the slope of the VCXO's frequency versus voltage characteristic changes too, and yet the compensating voltage VCOMP remains the same; this leads to the VCTCXO device becoming either over- or under-compensated at VCO voltages other than the one that the VCOMP was optimized at. The industry terms used to designate such deterioration of temperature compensation accuracy when external frequency control voltage level changes are “pulling skew”, “trimming skew”, “compensation tilt”, etc. It will be appreciated by persons skilled in the art that if the VCXO's frequency versus voltage characteristic slope was constant, the effect of the temperature compensating voltage VCOMP would be the same regardless of the frequency control voltage VCO value. In other words, a linear effective VCXO transfer function would eliminate “compensation tilt”.
Attempts are known to reduce such an undesirable deterioration of temperature compensation accuracy when the external frequency control voltage changes. For example, using the method for generating an input signal for a tunable circuit disclosed in the U.S. Pat. No. 6,549,055, it is possible to improve the linearity of the effective VCXO frequency versus voltage characteristic by pre-distorting the VCXO control signal using a polynomial approximation function to “substantially correct” for the non-linearities in the VCXO circuit. Such polynomial approximation methods are capable of reducing the non-linearity of the effective VCXO transfer characteristic to about 1%; any further non-linearity reduction by such methods is impossible due to the fact that the frequency versus voltage transfer function of a VCXO cannot be accurately defined by a simple mathematical expression such as a polynomial function.
The present invention offers a useful way of further reducing the compensation tilt errors in VCTCXO devices, by regenerating the VCXO control signal as a single, or a combination of a number of, signals generated as suitable functions of the initial frequency control signal.